Command-line plugin support

[mjpieters]

The proposed change

Allow (event or GUI) plugins to register command-line extensions; e.g. quodlibet --plugin=pluginname [args] would result in a specific method being called on the named plugin.

Exactly what type of plugin that supports this is an open question; command-line calls are perhaps events, but they are also a form of UI.

Benefits of this change

Personally, I have a need to automate QuodLibet beyond what is offered by the current crop of command-line options (which is very rich!). Specifically, I regularly switch out the default output device on my Macbook Pro, and on top of this add an Audio-Hijack pipeline. As a child I lost some of my hearing in one of my ears (I no longer have inner-ear bones on that side) and the audio-setup I use compensates for that. But only when I plug in my laptop to my home office dock; the switch is fully automated using Apple Script and command-line tools, except for QuodLibet. I always need to open the preferences panel, click on Apply on the Playback tab, to trigger a GStreamer pipeline rebuild, because GStreamer can't by itself detect that audio routing has changed. I can't automate this using Apple Script because GTK+ doesn't provide enough information to the OS to make this possible.

So, I'd love to be able to just write a plugin that calls player._rebuild_pipeline() on command. For now I think I'll resort to an event plugin that uses GLib.timeout_add to re-check audio settings repeatedly, but I'd much prefer to have a command-line option to trigger this.

Any issues this might cause

The biggest issue here is implementation work.

• Extending the plugin system to expose a method that can accept arbitrary command-line arguments
• Extending the CLI parser and Command infrastructure to support a new switch
• and the tests for this.

There is no performance cost beyond what a plugin might do with the call, but that's the case for any existing plugins.

There may be compatibility issues with how such a command would swallow all following command-line arguments.